A pro-democracy demonstrator burns a letter next to pictures of missing staff members of a publishing house and a bookstore, including Gui Minhai, a China-born Swedish national who is the owner of Mighty Current, Cheung Jiping, the business manager of the publishing house and Causeway Bay Books shareholder Lee Bo (L-R), during a protest to call for an investigation behind their disappearance, outside the Chinese liaison office in Hong Kong, China January 3, 2016. REUTERS/Tyrone Siu

BEIJING (Reuters) - Britain said on Tuesday that it was “deeply concerned” by reports of the disappearance and possible detention of five Hong Kong booksellers specializing in publications critical of China, one of whom is British, and urged the city’s commitment to press freedom.

A spokesperson for the British embassy in Beijing called on authorities in Hong Kong, a former British colony, and China to ascertain its citizen’s welfare and whereabouts. It did not identify the person.

“We are deeply concerned by reports about the disappearance and detention of individuals associated with the Causeway Bay Books bookstore in Hong Kong,” said the spokesperson, according to a statement from the British embassy in Beijing.

“We encourage the Hong Kong SAR government to honor its commitment to protecting the freedom of the press, and we hope the Chinese authorities will continue to make every effort to ensure that the environment in which the media and publishers operate in the Hong Kong SAR supports full and frank reporting.”

The remarks, referring to the “special administrative region” (SAR) of Hong Kong, came as British Foreign Secretary Philip Hammond began his two-day visit to China and after China’s foreign ministry warned that it would not tolerate outside interference in the affairs of Hong Kong.

Hong Kong returned to Chinese rule in 1997 with the guarantee of wide-ranging autonomy and an independent judiciary and media under a “one country, two systems” formula.

Lee Bo, 65, a shareholder of Causeway Bay Books, “vanished” last week, according to a missing person report filed by Lee’s wife on Thursday. Four associates involved in publishing or selling literature critical of Beijing have also gone missing in mysterious circumstances over the past few months.

The disappearances have stoked fears of mainland Chinese authorities using shadowy tactics that erode the “one country, two systems” formula under which Hong Kong returned to China.

Asked about the possibility of several of the booksellers holding British passports, Chinese Foreign Ministry spokeswoman Hua Chunying said that “Hong Kong is China’s domestic affair”.

“No foreign country has the right to interfere,” she said at a daily news briefing.

She said that “any Hong Kong residents of Chinese descent born in Chinese territory, including Hong Kong people,” were Chinese citizens.

Hong Kong leader Leung Chun-ying said on Monday there was no indication so far outside agents were involved in the disappearance of the booksellers, but that it would be “unacceptable” if any were.